Defense Secretary Jim Mattis sought to reassure skeptical lawmakers Thursday that National Guard troops deployed to the Mexico border would have a limited mission despite indications from President Trump that military personnel would remain there until he gets the $18 billion wall he wants built.

Mattis deflected questions about whether he planned to keep troops there as political leverage to fulfill Trump’s vision, saying a number of factors could lead to the end of the reinforcement mission. Instead, he described the deployment as an effort to “buy time” for the Department of Homeland Security and improve its enforcement capabilities.

The troops will operate under a policy of “no contact with the migrants,” Mattis said, and will support U.S. Customs and Border Protection, a subagency of DHS, as it heads into the months when migration flows typically increase.

“This is an anticipatory backing-up, so that Customs and Border Patrol can put more troops in the field,” he added.

Mattis said he could not yet say what the deployment’s cost would be, nor could he articulate a strategy beyond saying the troops would be used to support DHS.

The Pentagon has authorized funding for deployment of up to 4,000 National Guard members through the end of September. He said Thursday that 800 troops have been dispatched so far and that he expects to soon receive a request for another 700.

While on the border, they answer to the governors of the states to which they are assigned: California, Arizona, New Mexico and Texas. They will provide air support, maintain roads and other infrastructure, clear vegetation and assist with facility maintenance, in addition to operating surveillance systems.

“These forces will not involve themselves with the migrants themselves or have any law enforcement duties,” Mattis said. Troops conducting surveillance, for example, would pass along any intelligence gathered to Border Patrol agents, who in turn would handle any necessary seizures or arrests.

Mattis’s comments come as Democrats voice concern that Trump is deploying the National Guard not because states need military assistance but because the president has proved unable to secure funding for the border wall, a central campaign promise.

Ten Democrats on the House Armed Services Committee sent a letter Wednesday to Mattis opposing any attempt to redirect Pentagon funding to pay for a wall. The lawmakers warned that such budgetary maneuvering would take money away from funds the military needs to rebuild after years of budget caps.

“The National Guard should not be used as political props when the president does not get his way on something that can only be reconciled through Congressional appropriation,” said a spokesman for Rep. Anthony G. Brown (D-Md.), who signed the letter. “Guardsmen have jobs and families, and should only be called upon in times of war, crisis or imminent threat to our national security.”

Asked during Thursday’s hearing whether the National Guard would be tasked with building a wall, Mattis acknowledged he was looking at fencing off — or walling in — the Barry M. Goldwater Range in Arizona, a stretch of land along the border used by the Air Force and Marines to practice air-to-ground bombing.

“This is a safety consideration,” Mattis said. “I don’t care who they are, they are human beings, and I don’t want them wandering into a bombing range that is active.”

Customs and Border Protection has, in the past, raised concerns about migrants and drug smugglers crossing the range to enter U.S. territory, but as a matter of law enforcement — not necessarily safety.

It’s unclear how many safety incidents there have been on the Goldwater range. The military did not respond to requests for such data.

Some rights groups have documented migrants’ deaths on the range as a result of exposure to the elements, but not owing to a military mishap. “Over 30 years in the Marine Corps and the Border Patrol, I’ve never heard of anyone being injured in a bombing there,” said Border Patrol agent Vincent Dulesky, a supervisor reached by phone at the agency’s Yuma Sector, which includes the Goldwater Range.

A physical barrier already exists along much of the range’s border with Mexico, and signs are posted that warn of dangerous military activity. The George W. Bush administration ordered part of it fenced in with a physical and virtual barrier, citing some 8,600 people apprehended crossing into the country illegally via the range in 2006. The Obama administration canceled the project after the government spent nearly $1 billion to secure 53 miles of the roughly 2,000-mile border, including parts of the Goldwater range.

In testimony Thursday to the House Subcommittee on Defense Appropriations, the National Guard’s top commander, Gen. Joseph L. Lengyel, said Customs and Border Protection had started developing lists of tasks that the Guard would be able to perform in support of border agents, and that those tasks would be reviewed by Defense Department officials and passed along to National Guard units.

Arrests of illegal border-crossers fell to a 46-year low in 2017, but the numbers jumped last month, when U.S. agents made more than 50,000 apprehensions. It was the highest-one month total since Trump took office, and DHS officials said the surge of Central American families and teenagers amounts to a new “crisis” at the border.

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